A traditional rule of grammar states that you can only use the comparative degree when comparing two things. Thus you must say She is the older (not the oldest) of the two cousins. And you had better use better in similar comparisons: Which house of Congress has the better (not best) attendance record? Native speakers will recognize the natural sound of best in this sentence (and of oldest in the previous example) and will conclude that this traditional rule is often ignored in practice. In fact, many of our most celebrated writers use oldest, best, and other superlatives to compare two things. But don’t be surprised if someone objects when you follow suit. The traditional rules have many devoted adherents.
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Keep in mind, however, that there are certain fixed expressions, such as Put your best foot forward and May the best team win!, where you can’t go wrong using best.
